Polyolefin compounds having improved heat curing stability, method of improving heat aging stability therein, an electrical conductor insulated therewith and method of forming

ABSTRACT

Polyolefin compounds having improved resistance to oxidation and heat aging attributable to the incorporation therein of the combination of a zinc salt of a mercaptoimidazole with a sterically hindered phenol, a method of enhancing oxidation resistance and heat aging stability by means of the incorporation of the combination in polyolefins, an electrical conductor insulated therewith and method of forming.

This application is a continuation of Ser. No. 576,163 filed Jan. 30,1984, which in turn is a continuation of application Ser. No. 474,240filed Mar. 11, 1983 which in turn is a continuation-in-part ofapplication Ser. No. 965,716 filed Dec. 1, 1978, all now abandoned.

BACKGROUND OF THE INVENTION

The degrading effects upon the original physical state and integrity ofmany polymeric compositions, such as polyolefins, attributable tooxidation, and particularly under the accelerating effects of heat, arewell known disabling conditions or shortcomings of such materials whenused in services or products wherein long term performance is desired orrequired. For instance, to be economical and practical, polymericinsulations used for electrical conductors such as building wire,electrical motor or machinery power wires, or underground powertransmitting cables, must be enduring and must substantially retaintheir initial physical properties for effective and safe performanceover long terms of many years of service.

Moreover, as is also well known, elevated temperatures, a conditioncommonly encountered in many electrical devices or operations associatedtherewith, greatly accelerate the rate of oxidation degradation inpolymeric compositions. Thus, the problem of oxidation is commonlyreferred to as "heat aging" wherein the oxidizing reaction upon thepolymer material is quickened or intensified by increased temperatureconditions.

The debilitating oxidation deterioration of polymeric compositions, suchas physical embrittlement and cracking, is a subject of vast andintensive concern and remedial efforts in the polymer art and industryas is evident from the extensive amount of patent and other technicalliterature directed to this problem of oxidation or heat aging. Forexample, U.S. Pat. Nos. 3,819,410 and 3,979,180 deal with the problem ofpolymer oxidation and provide brief summaries of the state of the artand prior proposed solutions thereo, including the citation of a numberof relevant U.S. patents and their disclosed methods and techniques forresolving the same. Additional relevant patent literature comprises U.S.Pat. Nos. 3,296,189; 3,647,749, 3,717,610; 3,839,279; 3,886,114,3,904,705; 4,018,808 and 4,029,332.

As is evident from the prior art, and in particular the disclosures ofthe above patents, a substantial number of diverse and highly complexorganic compositions, commonly referred to as antioxidants, have beenfound to have some degree of stabilizing effect or retarding action uponthe deteriorating oxidation of polymers, and therefore have been widelyused to deal with this aspect of polymers. Additionally, it has beendiscovered that various combinations of such old antioxidant agents oradditives provide unusually enhanced or synergistically increasedstabilizing effects or inhibiting action in certain polymers.

SUMMARY OF THE INVENTION

This invention comprises the dscovery of a new and improved antioxidantsystem for curable and cured polyolefin polymers and compounds thereof,comprising the combination of a zinc slt of a mercaptoimidazole with asterically hindered di-tertiary butyl phenol, a method of stabilizingsuch polymers against oxidation and heat aging, electrical conditionsinsulated with cured polyolefins resistant to oxidation and stabilizedagainst heat aging and methods of forming same.

OBJECTS OF THE INVENTION

A primary object of this invention is to provide improved and lastingresistance to oxidation, or stabilized heat aging, in polyolefinpolymers or compounds thereof, and products comprising the sameincluding cable products.

A further object of this invention is to provide an antioxidant oroxidation inhibiting system which is highly effective for stabilizingheat aging in polyolefins and inhibiting oxidation therein over a widerange of temperature conditions and for greatly extended periods.

A still further object of this invention is to provide an effectivemeans of inhibiting oxidation or stabilizing heat aging in peroxidecrosslink cured polyolefins wherein the antioxidant or stabilizingadditives or agents do not distill off or otherwise become lost duringthe crosslink cure with heat or exposure to high temperatures.

Another object of this invention is to provide more effective andlasting antioxidant or stabilizing additives or agents which do notbleed from the polymer or are non-blooming, and do not adversely affectthe base polymer or its properties.

It is also an object of this invention to provide antioxidant orstabilizing additives or agents which are compatible with conventionalflame retarding systems comprising halogen-containing hydrocarbons withantimony oxide in polyolefin polymers.

It is additionally an object of this invention to provide longservice-life electrical conductors having cured polyolefin insulatons ofimproved and lasting heat aging stability or resistance to oxidation andto provide a method of forming such conductors.

BRIEF DESCRIPTION OF THE DRAWING

The drawing comprises a perspective view illustrating an insulatedelectrical conductor product embodying and produced according to thisinvention and produced by the method of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

In accordance with this invention, a high level of lasting resistance tooxidation, or heat aging stability, is achieved in cured polyolefinpolymers, or compounds thereof and electrical insulations producedtherewith, among other improved and advantageous attributes, byincorporating in such polyolefins the combination of a stericallyhindered di-tertiary butyl phenol with a zinc salt of mercaptoimidazole,and preferably zinc stearate.

The polyolefins of this invention comprise ethylene-containing polymers,including homopolymers such as polyethylene, copolymers of ethylene andother polymerizable materials, and blends of such polymers includingcopolymers. Typical copolymers of ethylene include, for example,ethylene-propylene copolymers and ethylene-propylene-diene terpolymers,ethylene-vinyl acetate, ethylene-ethyl acrylate and ethylene-methylacrylate.

The polyolefins, or compounds thereof, of this invention canadditionally comprise fillers, pigments, curing co-agents, fireretardants, and other conventional additives including preservativessuch as supplementary antioxidants, modifying agents such asplasticizers, mold releasing ingredients or lubricants and the likewhich are commonly compounded with polyolefins or typical productsthereof such as, for example, electrical insulations.

This invention also specifically applies to and includes all of theabove-referenced polyolefin polymers in a crosslink cured and thermosetstate, when the cure is effected by means of radiation or a chemicalcuring agent comprising a heat-activatable organic peroxide crosslinkingagent such as disclosed in U.S. Pat. Nos. 2,888,424; 3,079,370;3,086,966; and 3,214,422. Suitable peroxide crosslink curing agentscomprise organic tertiary peroxides characterized by at least one unitof the structure: ##STR1## which decomposes at a temperature above about146° C. (295° F.) and thereby provide free radicals. Preferred peroxidefor curing polyolefins is di-α-cumyl peroxide; and other apt peroxidescomprise the tertiary diperoxides such as 2,5-dimethyl-2,5-di(t-butylperoxy) hexane) hexyne-3, and the like diperoxy and polyperoxidecompounds.

The sterically hindered phenols for use in this invention, incombination with the zinc salt of a mercaptoimidazole, consist ofconventional sterically hindered di-tertiary butyl mono and poly2,6-di-t-butyl phenols. Preferred hindered phenol antioxidants for usein the practice of this invention comprise: Tetra kis[methylene-3(3',5'-di-tert-butyl-4'-hydroxy phenyl)propionate]methane,sold under the trade designation Irganox 1010 by Ciba-Geigy; thioesterbis hindered phenol, sold under the trade designation Irganox 1035 byCiba-Geigy; octadecyl 3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate, sold under the trade designation Irganox 1076 byCiba-Geigy and, 1,3,4-trimethyl-2,4,6-tris(3',5'-di-tert-butyl-4'-hydroxy phenyl) benzene, sold under the tradedesignation Antioxidant 330 by Ethyl Corp. Other hindered phenolantioxidants are listed in U.S. Pat. No. 3,979,180.

The zinc salt of a mercaptoimidazole component of the combination ofthis invention comprises a zinc salt of a 2-mercaptobenzimidazole soldunder the trade designation Vulkanox ZMB by Mobay Chemical, or a zincsalt of 2-mercaptotolylimidazole sold under the trade designation VanoxZMTI by R. T. Vanderbilt or Vulkanox ZMB-2 by Mobay Chem.

The combination of a zinc salt of a mercaptimidazole with a stericallyhindered di-tertiary butyl phenol antioxidants of this invention can bemixed and combined with a polyolefin material or compound thereof, bymeans of any conventional compounding method or apparatus, such asworking in a Banbury mixer or on a two roll rubber mill. Preferably allingredients of the compound formulation, except those which aresensitive to the relatively moderate mixing temperatures of about 150°C. (300°F.) to about 205° C. (400°F.), such as a heat decomposableorganic peroxide curing agent, are combined and initially admixedtogether at a temperature sufficient to often and plasticize theparticular polyolefin polymer ingredients. Following the attainment of asubstantial uniformity of the initially admixed ingredients, thetemperature of the admixed batch is reduced below the deomposition levelof the particular peroxide curing agent used, or other heat sensitiveingredients to be added, and the curing agent or other heat sensitiveingredients are then introduced and dispersed preferably uniformlythroughout the mix.

The proportions of the antioxidant ingredients of the novel stabilizingcombination of this invention added to the polyolefin compounds can bevaried and depends primarily upon the level of heat aging stability oroxidation inhibition desired or required in the cured polymer materialor product thereof. However, with the higher degree or levels of heataging stability or oxidation inhibition provided by the novelcombination of this invention comprising the essential stericallyhindered phenol with a zinc salt of a mercaptoimidazole, typical amountsof the essential combination comprise about 1 to about 6 parts by weightof the hindered phenol per 100 parts by weigh of he polyolefin polymer,and about 1 to about 12 parts by weight of the zinc salt of amercaptoimidazole per 100 parts by weight of the polyolefin polymer. Ina preferred embodiment, both components of the novel combination ofessential ingredients which are responsible for the improved and lastingheat againg, stability or inhibited oxidation, are included or added inthe polymer in a ratio to each other of approximately 1 part of thehindered phenol per 2 to 4 parts of the zinc salt of a mercaptoimidazoleby weight, for example about 4 to 8 parts by weight of the zinc salt ofa mercaptoimidazole with about 2 to 4 parts by weight of the hinderedphenol per 100 parts by weight of the polymer generally provides optimumheat aging stability. Although greater amounts of the agents can beused, the benefits derived from such larger amounts thereof aregenerally not proportional thereto.

Additionally, ancillary or supplemental antioxidants can be included inthe overall polyolefin compounds in conventional amounts in addition tothe particular novel and essential combination of this invention.

The zinc stearate of the preferred embodiment of this invention isincluded in amounts of up to about 6 parts by weight based on 100 partsby weight of the polymer. For example, about 1 to about 6 parts byweight thereof.

As noted hereinbefore regarding the heat aging stabilizing systems ofthis invention, the antioxidant combination is compatible with manyflame retarding systems and can be included with conventional flameretardants in polyolefin compounds. Such typical flame retarding systemscomprise halogen-containing organic compounds or materials which areused with antimony compounds or their equivalents, including thehalogenated hydrocarbons of U.S. Pat. No. 2,480,298. For example,conventional halogenated hydrocarbons for flame resistance comprisechlorinated paraffin, chlorinated propanes, chlorinated propylenes,hexachloroethane, chlorinated polythene, chlorinated polyisobutylene,polyvinyl chloride, polyvinylidene chloride, after-chlorinated polyvinylchloride, chlorinated polyphenyls, chlorinated naphthalenes,hexachlorobenzene, chlorinated idenes, chlorinated polystyrenes,chlorinated diphenyl alkanes, chlorinated polyphenyl ethers, and theirbrominated or other halogenated equivalents, such as halogenatedalkanes, aromatics, polyaromatics and hetrocyclics. Also included asconventional halogenated hydrocarbons are proprietrary halogenated flameretardants such as Hooker Chemical Company's Dechlorane Plus 25 or Plus515, Diamond Alkali Company's Chlorowax, and similar products.Dechlorane Plus 25 is the condensation product of 2 moles ofhexachlorocyclopentadiene with 1 mole of cyclooctadiene.

Halogen-containing flame retardants can be included in substantialamounts of up to about 70 parts by weight per 100 parts by weight of thepolyolefin polymer, and preferably also included in an antimony oxidecompound which can be present in amounts of up to about 35 parts byweight per 100 parts by weight of the polyolefin polymer.

The following examples and data derived therefrom demonstrate theimproved effects of the novel combinations of this invention in typicalcrosslink cured polyolefin compounds formulated for use as an electricalinsulation with respect to the same polymer compositions as standards,including single antioxidants of the same compositions or differentcombinations of other conventional antioxidants, or other variations inthe components. The polymeric insulating composition formultions of thestandards and of the examples of this invention are all given inrelative parts by weight, and the compositions of the standards andexamples of the invention in each evaluation or series of tests were allprepared, cured and tested under identical conditions.

The following comparative evaluations demonstrate the pronounced andsignificant synergistic effect upon heat aging in polyolefin compoundsattributable to the antioxidant system of this invention, comprising theunqiue combination of the sterically hindered phenol with a zinc salt ofa mercaptoimidazole in relation to the same antioxidants used alone asstandards. In the tests applied to the compositions comprising thestandards and examples of the invention, the original physicalproperties and hot modulus determinations thereof provide a measure ofthe state or degree of cure of the polymer compositions. The heat agingtest was carried out at a relatively high and debilitating temperaturelevel of 180° C. in a air oven for accelerated heat aging withmeasurement of tensile and elongation made at 7 day, 14 day and 21 dayperiods of exposure to such temperature and reported as percentretention of original tensile and elongation. The compositions of thestandards and examples of the invention were all press cured for 20minutes at 350° F. The ingredients of the compositions of the standardsand of the invention, and their properties elevant to heat aging aregiven in the following Table I.

                                      TABLE I                                     __________________________________________________________________________                           STANDARDS         EXAMPLES                                                    A     B     C     I     II                             __________________________________________________________________________    COMPONENTS, PARTS BY WEIGHT                                                   Polyethylene-Ethylene vinyl acetate copolymer                                                        100   100   100   100   100                            (72% polyethylene - 18% ethylene vinyl                                        acetate) (Alathon 3170 du Pont)                                               Zinc salt of 2-mercaptobenzimidazole                                                                 --    6     --    4     --                             (Vulkanox ZMB Mobay)                                                          Tetra Kis [methylene-3(3',5'-di-tert-butyl-                                                          --    --    6     2     2                              4'-hydroxy phenyl) propionate] methane                                        (Irganox 1010 - Ciba-Geigy)                                                   Zinc stearate          --    --    --    --    2                              Curing coagent (SR-350 Sartomer Co.)                                                                 4     4     4     4     4                              Trimethylol propane tri methacrylate                                          Dicumyl peroxide curing agent                                                                        2.5   2.5   2.5   2.5   2.5                            (Dicup-R - Hercules)                                                          PROPERTIES                                                                    Original Tensile, psi  2258  2912  2568  3515  4023                           Original Elongation, percent                                                                         326   430   438   497   524                            Hot Modulus, psi       90@40%                                                                              106@75%                                                                             79@70%                                                                              82@100%                                                                             109@7.3                        Toluene Extraction, percent                                                                          3.5   14.4  14.2  13.3  10.6                           7 Days - 180° C.                                                       Percent Retention, Tensile                                                                           17    120   75    102   105                            Percent Retention, Elongation                                                                        4     104   72    101   95                             14 Days - 180° C.                                                      Percent Retention, Tensile                                                                           12    36    28    108   85                             Percent Retention, Elongation                                                                        1     21    11    92    77                             21 Days - 180° C.                                                      Percent Retention, Tensile                                                                           12    32    22    41    71                             Percent Retention, Elongation                                                                        0     4     3     46    59                             __________________________________________________________________________

As is apparent from the data, the composition of Example 1 including theantioxidant system of a combination of this invention retained 45% ofelongation after 21 days at 180° C., compared to the less than 5%elongation retention of the composition of the standards with a likequantity of either antioxidant alone. The added inclusion of zincstearate in the system in Example II enhances both the extent of thecure and the effect of the antioxidant system of this invention toprovide a further improvement in both the original and heat agedphysical properties as evidenced by the lower toluene extraction andhigher hot modulus.

The base polymeric composition or compound used for the samples of boththe standards and examples of the invention in all following comparativeevaluations consists of the formulation set forth below in parts byweight based upon 100 parts by weight of the polymer content. The basiccomposition formulation included a commercial flame retardant,Dechlorane Plus 25, and antimony oxide to provide a typical flameresisting system. This basic composition formulation providing theStandard, was modified as indicated in each instance in the Tables.

    ______________________________________                                                                 PARTS BY                                             INGREDIENT               WEIGHT                                               ______________________________________                                        Polyethylene-ethylene vinyl acetate                                                                    100                                                  copolymer (72% polyethylene - 18% EVA                                         copolymer) (Alathon 3170 - du Pont)                                           Tetra Kis [methylene-3(3',5'-di-tert-butyl-                                                            2                                                    4'-hydroxy phenyl) propionate] methane                                        (Irganox 1010 - Ciba Geigy)                                                   Zinc salt of 2-mercaptoimidazole                                                                       4                                                    (Vulkanox ZMB - Mobay Chemical)                                               Zinc stearate            2                                                    Condensation product of 2 moles hexachloro-                                                            35                                                   cyclopentadiene and 1 mole of cyclooctadiene                                  (Dechlorane Plus 25 - Hooker Chemical)                                        Antimony oxide           17.5                                                 Trimethyl propane tri methacrylate curing                                                              4                                                    coagent (SR-350 - Sartomer Co.)                                               Dicumyl peroxide crosslink curing agent                                                                2.5                                                  (Di Cup - Hercules)                                                           ______________________________________                                    

The following evaluations of the effects upon heat aging in a polyolefincompound attributable to combinations of various antioxidants or typesthereof demonstrates the selectivity and distinctiveness of theparticular combinations of the antioxidant system of this invention.Tables II and III demonstrate the relative resistance to heat agingprovided by several different combinations of similar and unlike typesof antioxidants, comprising the combinations of antioxidants of thisinvention and combinations of antioxidants outside of the invention.Table II tests and demonstrates the effects of the use of severaldifferent imidazole or thiazole antioxidants combined with a stericallyhindered phenol (Irganox 1010) antioxidant as a substitute for theVulkanox ZMB of Examples I and II, included therewith in the equivalentmolar basis as the 4 parts by weight of the Vulkanox ZMB per 100 partsby weight of the polymer, and combined with the 2 parts by weight of thesterically hindered phenol antioxidant Irganox 1010. Table IIIalternatively tests and demonstrates the effects of the use of severalphenolic types of antioxidants combined with a zinc salt of amercaptoimidazole (Vulkanox ZMB) antioxidant as a substitute for theIrganox 1010 of Examples I and II, included therein in the equivalentmolar basis as the 2 parts by eight of the Irganox 1010 per 100 parts byweight of the polymer, and combined with 4 parts by weight of the zincsalt of mercaptoimidazole Vulkanox ZMB.

                                      TABLE II                                    __________________________________________________________________________                                           21 Days - 180° C.                                    Parts             Air Oven Aging                         Irganox 1010* Plus   By  Original Physicals                                                                     Hot  Percent Retention                      Sample                                                                             Antioxidant Agent Of:                                                                         Weight                                                                            Tensile                                                                            Elong.                                                                            Modulus                                                                            Tensile                                                                            Elong.                            __________________________________________________________________________    Standard                                                                           Zinc salt of 2-mercaptobenz-                                                                  4.0 2967 473 81   32   30                                     imidazole (Vulkanox ZMB)                                                      (Mobay)                                                                  1    Zinc salt of 2-mercaptotolyl-                                                                 4.37                                                                              3037 488 63   36   39                                     imidazole (Vanox ZMTI)                                                        (R. T. Vanderbilt)                                                       2    2-mercaptobenzimidazole                                                                       1.98                                                                              2923 503 57   21   3                                      (Vulkanox MB) (Mobay)                                                    3    2-mercaptotolylimidazole                                                                      2.17                                                                              2875 498 74   20   2                                      (Vanox MTI) (R. T. Vanderbilt)                                           4    2-mercaptobenzothiozole                                                                       2.20                                                                              1925 520 12   28   1                                      (Captax) (R. T. Vanderbilt)                                              5    Zinc salt of 2-mercaptobenz-                                                                  5.24                                                                              2531 467 44   32   6                                      othiozole (ZMBT Waxed)                                                   __________________________________________________________________________     *Irganox 1010  Tetra kis [methylene3(3',5di-tert-butyl-4hydroxy phenyl)       propionate] methane                                                      

As is apparent from the data of Table II, only the zinc salts of themercaptoimidazoles, when combined with the sterically hindereddi-tertiary butyl phenol antioxidant, produce a pronounced ksynergisticeffect in controlling heat aging, and only the imidazoles providedeffective protective characteristics for high temperature service.

                                      TABLE III                                   __________________________________________________________________________                                          21 Days - 180° C.                                    Parts             Air Oven Aging                          Vulkanox ZMB* Plus  By  Original Physicals                                                                     Hot  Percent Retention                       Sample                                                                             Antioxidant Agent Of:                                                                        Weight                                                                            Tensile                                                                            Elong.                                                                            Modulus                                                                            Tensile                                                                            Elong.                             __________________________________________________________________________    Standard                                                                           Tetra kis [methylene-3(3',5'-                                                                2.0 2967 473 81   32   30                                      di-tert-butyl-4'-hydroxy                                                      phenyl) propionate] methane                                                   (Irganox 1010) (Cibi-Geigy)                                              1    Bis(4-t-butyl-3 hydroxy-2,6-                                                                 1.84                                                                              2217 430 53   32    8                                      benzyl) dithioterephthalate                                                   (Cyanox 1729) (Amer.                                                          Cyanamid)                                                                2    Octadecyl 3-(3',5'-di-tert-                                                                  3.61                                                                              2609 501 41   39   37                                      butyl-4'-hydroxy phenyl)                                                      propionate (Irganox 1076)                                                     (Cibi-Geigy)                                                             3    Thioester bis hindered phenol                                                                2.18                                                                              2931 497 57   35   35                                      (Irganox 1035) (Cibi-Geigy)                                              4    (Mark 1589) (Argus Chemical                                                                  2.04                                                                              2678 460 86   53   59                                      Corp.)                                                                   5    4,4'-thio bis (3-methyl-6-                                                                   1.22                                                                              2695 485 57   28   16                                      tert-butyl phenol)                                                            (Santonox R) (Monsanto)                                                  6    4,4'-thio bis-(6-tert-                                                                       1.22                                                                              2584 485 62   29   1                                       butyl-m-cresol)(Santowhite                                                    Crystals) - Monsanto                                                     7    (CAO-44)       1.22                                                                              2597 482 56   27    5                                      Ashland Chemical Company                                                 8    (Goodrich TTT) 2.20                                                                              2445 454 106  38   17                                      B. F. Goodrich                                                           9    1,1,3-trio(2'-methyl-4'-                                                                     1.23                                                                              2923 484 72   26   10                                      hydroxy-5'-tert-butyl phenyl)                                                 butane (Topanol CA)                                                           ICI United States, Inc.                                                  __________________________________________________________________________     *Vulkanox ZMB -- zinc salt of 2mercaptobenzimidazole                     

The data of Table III demonstrates that only the sterically hindereddi-tertiary butyl phenols, when combined with the zinc salts of themercaptoimidazole antioxidants, produce a pronounced synergisic effectin controlling heat aging and permanence.

The following Table IV illustrates the practice of this invention of thecombination of a zinc salt of a mercaptoimidazole with a stericallyhindered phenol as set forth in the above base polymer compositionformulation, with several different polyolefin polymer materials inplace of the Alathon 3170 polymer as the standard.

                                      TABLE IV                                    __________________________________________________________________________                                    Deforma-                                                                             21 Days - 180° C.                                               tion Re-                                                                             Air Oven Aging                                       %   Original Physicals                                                                     Hot  sistance                                                                             Percent Retention                      Sample                                                                             Polymer  EVA Tensile                                                                            Elong.                                                                            Modulus                                                                            @180° C. %                                                                    Tensile                                                                            Elong.                            __________________________________________________________________________    Standard                                                                           Alathon 3170                                                                           18  2857 488 60   24.6   44   40                                     (du Pont)                                                                1    Alathon 3130                                                                           12  2721 444 58   17.2   45   43                                     (du Pont)                                                                2    DQD - 1868                                                                             18  2759 460 60   18.4   58   52                                     (Union Carbide)                                                          3    UE - 632 14  2525 448 58   23.8   34   15                                     (U.S. Industries)                                                        4    UE - 637 9   2694 437 67   19.8   44   26                                     (U.S. Industries)                                                        5    UE - 635 9   2181 433 55   26.9   52   31                                     (U.S. Industries)                                                        6    EH - 497 0   2670 434 112  14.0   51   28                                     (Cities Service)                                                              (polyethylene)                                                           __________________________________________________________________________

As is evident from the data, the polyolefin composition of thisinvention, containing the combination of the specified stericallyhindered phenol with a zinc salt of a mercaptoimidazole is significantlymore stable than those containing combinations of similar antioxidants.

The cured, heat aging stabilized or oxidation resistant polyolefinpolymer, or compounds thereof, of this invention are particularly usefulmaterials for dielectric insulation for electrical conductors such aswire and cable. A typical insulated electrical conductor product of thisinvention is illustrated in the drawing. Referring to the drawing, aninsulated product 10 comprising a metallic conductor 12, composed of asingle strand as shown, or a bundle of individual strands, having anoverlying electrically insulating covering 14 of a polymeric compositionaccording to the present invention and which is highly resistant tooxidation and stabilized against heat aging.

Further, the present invention includes the distinct and novel methodsby which products such as those described immediately above areprepared.

Among the novel methods of the present invention are the following:

A. A method of producing an electrical conductor insulated with a curedpolyolefin compound having improved and lasting heat aging stability,comprising combining with an organic peroxide crosslink curablepolyolefin polymer the combination of a sterically hindered ditertiarybutyl phenol in an amount of about 1 to about 6 parts by weight per 100parts by weight of the polyolefin polymer and at least one zinc salt ofa mercaptoimidazole selected from the group consisting of a zinc salt of2-mercaptobenzimidazole and a zinc salt of 2-mercaptototolylimidazole inan amount of about 2 to about 12 parts by weight per 100 parts by weightof the polyolefin polymer, applying the curable polyolefin polymercombined with said combination to an electrical conductor, and crosslinkcuring the curable polyolefin polymer with an organic peroxide crosslinkcuring agent.

B. A method of producing an electrical conductor insulated with a curedethylene-containing polymer compound having improved and lasting heataging stability, comprising combining with an organic peroxide crosslinkcurable ethylene-containing polymer selected from the group consistingof polyethylene, copolymers of ethylene and other polymerizablematerials, and blends of polyethylene and other polymers, and mixturesthereof, the combination of a sterically hindered di-tertary butylphenol in an amount of about 1 to about 6 parts by weight per 100 partsby weight of the ethylene-containing polymer and at least one zinc saltof a mercaptoimidazole selected from the group consisting of zinc saltof 2-mercaptobenzimidazole and a zinc salt of 2-mercaptotolylimidazolein an amount of about 2 to about 12 parts by weight per 100 parts byweight of the ethylene-containing polymer, applying the curableethylene-containing polymer combined with said combination to anelectrical conductor, and crosslink curing the curableethylene-containing polymer with an organic peroxide crosslink curingagent.

C. A method of producing an electrical conductor insulated with a curedethylene-containing polymer having improved and lasting heat agingstability, comprising combining, in approximate relative parts byweight, the ingredients of:

Ethylene-containing polymer: 100.0

Sterically hindered di-tertiary butyl phenyl: 1-6

At least one zinc salt of a mercaptoimidazole selected from the groupconsisting of 2-mercaptobenzimidazole and 2-mercaptotolylimidazole: 2-12

Zinc stearate: 1-6

Halogen-containing organic compound: 0-70

Antimony oxide: 0-35

Peroxide crosslink curing agent: 2-6

applying the crosslink curable polyethylene combined with saidingredients to an electrical conductor, and crosslink curing the curableethylene-containing polymer.

Although the invention has been described with reference to certainspecific embodiments thereof, numerous modifications are possible and itis desired to cover all modifications falling within the spirit andscope of this invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:
 1. A polyolefin polymer composition which may be curedby cross-linking with an organic peroxide curing agent comprising apolyolefin polymer and from about 1 to 6 parts by weight per 100 partsby weight of said polyolefin polymer of a sterically hindereddi-tertiary butyl phenol and from about 1 to about 12 parts by weight ofsaid polyolefin polymer of at least one zinc salt of mercaptoimidazoleselected from the group consisting of a zinc salt of2-mercaptobenzimidazole, a zinc salt of 2-mercaptotolylimidazole andmixtures thereof.
 2. The polyolefin polymer of claim 1 wherein saidphenol is present in a ratio of about 1 part by weight to every about 2to about 4 parts by weight of the zinc salt.
 3. The polyolefin polymerof claim 1 which contains a flame retardant composition.
 4. The polymerof claim 3 wherein said flame retardant composition is a combination ofa halogen-containing organic compound and an oxide of antimony.
 5. Thecured cross-linked polymer of claim
 1. 6. The polyolefin polymer ofclaim 1 com;prising at least one ethylene-containing polymer selectedfrom the group consisting of polyethylene, copolymers of ethylene andother polymerizable materials and blends of polyethylene and otherpolymers.
 7. The cured cross-linked polymer of claim
 6. 8. Thepolyolefin polymer of claim 1 containing from about 1 to about 6 partsby weight of zinc stearate per 100 parts by weight of polymer.
 9. Thecured cross-linked polymer of claim
 8. 10. The polyolefin polymer ofclaim 1 having an improved heat-aging property comprising by weight therelative amounts of the following materials:(a) ethylene containingpolymer: Parts 100 (b) sterically hindered di-tertiary butyl phenol: 1-2(c) said at least one zinc salt: 1-12 (d) zinc stearate: 1-6 (e)halogen-containing organic compound: 0-70 (f) antimony oxide: 0-35 (g)organic peroxide curing agent: 2-6.
 11. A polyolefin polymer compositionwhich may be cured by cross-linking with an organic peroxide curingagent comprising a polyolefin polymer, a sterically hindered di-tertiarybutyl phenol and at least one zinc salt selected from the groupconsisting of the zinc salt of 2-mercaptobenzimidazole, the zinc salt of2-mercaptotolylimidazole and mixtures thereof.
 12. A polyolefin polymercomposition as defined in claim 2 wherein the ratio of the zinc salt tothe sterically hindered di-tertiary butyl phenol is about 2 to 4 partsby weight of the zinc salt to one part by weight of the stericallyhindered di-tertiary butyl phenol by weight.
 13. A polyolefin polymercomposition as defined in claim 12 wherein the composition includes 4parts by weight of zinc-2-mercaptobenzimidazole and 2 parts by weight oftetrakis(methylene-3(3'-5'-di-tert-butyl-4'-hydroxyphenyl)propionate)methane per 100 parts by weight of polyolefin polymer.14. A method of improving and prolonging the heat aging stability ofcured polyolefin polymer compounds, comprising combining with a curablepolyolefin polymer the combination of a sterically hindered di-tertiarybutyl phenol in an amount of about 1 to about 6 parts by weight per 100parts by weight of the polyolefin polymer and at least one zinc salt ofa mercaptoimidazole selected from the group consisting of a zinc salt of2-mercaptobenzimidazole and a zinc salt of 2-mercaptotolylimidazole inan amount of about 1 to about 12 parts by weight per 100 parts by weightof the polyolefin polymer, and cross-linking the polymer with an organicperoxides curing agent.